FIG. 1 is a schematic diagram of the frame structure of time division duplex (TDD for short) mode in the LTE system. As shown in FIG. 1, in such a frame structure, a 10 ms (307200 Ts, 1 ms=30720 Ts) radio frame is divided into two half-frames, each of which is 5 ms (153600 Ts) long and comprises 8 normal time slots with a length of 0.5 ms and 3 special time slots, i.e., a downlink pilot time slot (DwPTS for short), a guard period (GP for short) and an uplink pilot time slot (UpPTS for short), and the total length of these 3 special time slots is 1 ms (30720 Ts). Subframe 1 is always consisted of 3 special time slots; when there are 2 downlink-to-uplink conversion points in 10 ms, subframe 6 is consisted of 3 special time slots; in other cases, subframe 6 only comprises a DwPTS (the length of the DwPTS is 1 ms at this time), and other subframes are consisted of 2 normal time slots.
In the above frame structure, subframes 0, 5 and the DwPTS are used for downlink transmission, subframe 2 and the UpPTS are used for uplink transmission. When there are 2 downlink-to-uplink conversion points in 10 ms, subframe 7 is also used for uplink transmission.
In the TDD mode in the LTE system, physical random access channel (PRACH for short) can be classified into two types:
The first type is transmitted in general uplink subframes (not including subframes of special time slots), and comprises 4 kinds of PRACHs as listed below:
(1) Preamble format 0: occupying 1 uplink subframe, the length of the cyclic prefix (CP) being 3168 Ts, and the length of the preamble being 23576 Ts;
(2) Preamble format 1: occupying 2 uplink subframes, the length of the cyclic prefix (CP) being 21024 Ts, and the length of the preamble being 24576 Ts;
(3) Preamble format 2: occupying 2 uplink subframes, the length of the cyclic prefix (CP) being 6240 Ts, and the length of the preamble being 2×24576 Ts;
(4) Preamble format 3: occupying 3 uplink subframes, the length of the cyclic prefix (CP) being 21024 Ts, and the length of the preamble being 2×24576 Ts;
The second type is transmitted in the UpPTS and comprises one kind of PRACH, Preamble format 4, of which the length of the CP is 448 Ts and the length of the preamble is 4096 Ts;
In the frequency domain, all of the various PRACHs mentioned above occupy 6 resource blocks (RB), each RB comprises 12 sub-carriers, and each sub-carrier has a bandwidth of 15 kHz.
Upon accessing a system, a mobile phone needs to perform downlink synchronization, then demodulate the broadcast channel to obtain the configuration parameters of the PRACH, finally perform uplink synchronization through the PRACH, and establish a link to the base station. Herein, the configuration parameters of the PRACH in the TDD mode comprises density (how many PRACHs are usable in a unit of time, for example, D=2 PRACHs/10 ms, which means that there are 2 PRACHs usable in 10 ms), preamble format, version number.
Wherein same format, density with different version number means that preamble formats are the same, the numbers of PRACHs in a unit of time are the same, but the locations of these PRACHs in the frequency domain or time domain are different. The purpose that a plurality of versions are set for the PRACHs of the same format and same density and different cells use different versions is to scatter the PRACHs of different cells managed by the same base station in the time domain, so as to make individual cells managed by the same base station request for processing PRACHs at different time as much as possible, thus avoid the case that the base station is over busy at some time but has no data to process at other time. In addition, for the PRACH of preamble format 4, since no data is sent in the UpPTS, different cells use different versions, the PRACH of each cell has different time domain or frequency domain location, which can reduce the inter-cell interference of the PRACH.
During the process of uplink synchronization, the mobile phone obtains the time domain locations and the frequency domain locations of the PRACHs usable for the present cell using the PRACH configuration parameters by a certain algorithm. In the prior art, no reasonable mapping algorithm for the time domain location and the frequency domain location is provided, which results in problems that the PRACHs which need to be processed by the same base station are distributed unevenly in the time domain, and inter-cell interference of the PRACH of the second type is comparatively severe.